Damage assessment of structural components used in industries like power, petrochemical, steel etc. is required for prevention of premature failure. Carburization is one of the causes for the failure of steel components in petrochemical industries. Carburization occurs at elevated temperature in the presence of carbon rich gases. Associated phenomenon such as metal dusting, microstructure alteration and brittleness reduce the service life of the industrial components. Thus, evaluation and monitoring of carburization is necessary to avoid catastrophic failure of components. The process of high temperature carburization involves the steps of formation of carbon layer on the surface, inward diffusion of dissolved carbon into the metal, reaction of carbon with carbide forming elements to form carbides of the type Cr23C6 and C7C3. The carbides and the austenitic steel are paramagnetic in nature. However, the formation of these chromium carbides leads to depletion of chromium in the matrix. Consequently there is enrichment of Fe and Ni in the matrix and the material becomes ferromagnetic in nature. PZT-based sensors are widely used to determine the flaws in structural components. Inductive sensors using ferrite cores are also extensively used for nondestructive testing (NDT). Magnetic sensors with high sensitivity have been investigated for some years to improve the performances of sensing device. Anisotropic magneto-resistance (AMR), giant magneto-resistance (GMR), fluxgate and SQUID sensors have been explored not only for the flaw identification but also evaluation of damages that occur prior to the formation of cracks. Attempts have been made to develop sensing devices using GMI materials.
Austenitic stainless steel have high amount of chromium (16 to 20%) and nickel (8 to 10%). Due to high nickel and chromium content the steel remains in austenitic phase even in the room temperature. Hence the material exhibits paramagnetic property. Carburization results in the formation of chromium carbides in the austenitic stainless steel. Chromium carbides are formed because chromium has more affinity to form Carbides than nickel and iron which are present in the material. The carbides generally formed are Cr23C6 and Cr7C3. Due to the formation of these carbides small chromium depleted areas is formed near to carbide sites. These chromium depleted areas will have high relative concentration of iron and nickel. Due to the increase in the concentration of iron and nickel these areas will transform from paramagnetic to ferromagnetic state.
GB 1517096 discloses a device for monitoring carburization by measuring permeability in which the energizing coil is excited at a certain frequency and the e.m.f induced in detecting coil coupled to the energizing coil is measured. The method is not suitable as the ferromagnetic oxides formed in outer surface of tubes also influence the measurement. EP 81304158.9 discloses measurement of carburization in furnace tubes using the method of differential permeability technique. However, such technique has limitation owing to frequency selection criteria to meet the desired penetration depths.
Ferromagnetic behavior of stainless steel due to carburization is studied by various authors. In these studies, the level of carburization of the samples was determined by nondestructive magnetic flux density measurements before they were removed from the tubes. This technique measures the magnetic flux density near the external surface of the tubes by means of a magnetoresistive sensor biased by a small ferrite magnet.